Monday, November 12, 2012

Winds Drive Growth in Antarctic Sea Ice

Northward winds are driving the record growth of winter sea ice around Antarctica, which stands in contrast to the extensive melting of the Arctic sea ice in recent years, scientists reported Sunday.

Their new research, based on 19 years of daily ice-motion measurements recorded by four satellites of the U.S. Defense Meteorological Satellite Program, highlights how geography, weather and climate patterns are affecting the planet's polar regions in different ways.

Their analysis documented for the first time that long-term changes in the drift of annual sea ice around Antarctica were strongly affected by winds. The area of ocean covered by sea ice grew markedly in regions where the prevailing winds spread out the loosely compacted ice floes, they reported. It shrank in areas where the wind blew the floating ice up against the shore.

The researchers at NASA's Jet Propulsion Laboratory in California and the British Antarctic Survey reported their work Sunday in Nature Geoscience.

"We have evidence now that the wind is driving the ice cover," said JPL senior research scientist Ron Kwok, who led the study. "The expansion and contraction of ice around the continent is largely explained by wind forces, which is very different in the Antarctic than in the Arctic."

Broadly speaking, the Earth's polar regions are mirror opposites.

The Arctic Ocean is largely landlocked, surrounded by North America, Greenland and Eurasia, which limits the amount of sea ice there could be no matter which way the wind is blowing. Earlier this year, scientists reported that the extent of summer sea ice in the Arctic was the lowest since 1979, when satellite monitoring there began.

By contrast, Antarctica—the world's coldest and windiest continent—is covered by an ice cap two miles thick and surrounded by the Southern Ocean. The annual growth of sea ice around Antarctica is the largest seasonal event on the planet.

With the onset of the Southern Hemisphere winter in March, the ice expands at 22 square miles a minute. In 1992, the direction of the drifting sea ice changed, with the spread of the ice doubling in some regions, the satellite measurements showed. Earlier this year, Antarctica's sea ice reached a record expanse of 7.49 million square miles, before the spring thaw began.

In a separate study made public last month, climate scientists at NASA's Goddard Space Flight Center in Maryland reported that winter sea ice surrounding Antarctica has been increasing by about 6,600 square miles every year—an area larger than Connecticut—during the same decades that the Arctic summer sea ice has been shrinking.

The researchers didn't identify what was driving wind patterns around Antarctica. Generally, the annual ozone hole over Antarctica has strongly affected wind circulation throughout the Southern Hemisphere, studies have shown. Wind patterns around Antarctica also are linked to larger climate cycles such as El Niño.

"The larger connection to global climate change and warming is more difficult to say," Dr. Kwok said. "We don't understand that yet."